Apparatus for measuring minute forces

ABSTRACT

An apparatus for measuring forces across a flexible member is provided comprising a non-extensible element suspended between a pair of supports. The non-extensible element is operably coupled to a flexible member for translating forces applied to the non-extensible element to the flexible member. A pair of equally massed weight dishes are suspended from said non-extensible element on opposite sides of the flexible member. By incrementally adding weights to one of the weight dishes, the flexible member is incrementally displaced. An Eddy Current System operably communicates with the flexible member for detecting the displacement. The displacement and load are used to calibrate a stress-strain curve of the flexible member.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention generally relates to force sensors and moreparticularly, force sensors capable of measuring extremely small forces.

2. Discussion

As is generally known in the art of force testing, sensors are utilizedfor measuring forces in horizontal and vertical directions. Generally,these sensors are first calibrated over an expected range prior to thereuse in measuring unknown forces. To accomplish this, conventional testinstruments employ load cells or strain gauges for measuring a forcedirected on a flexible member. The strain gauges and load cellstransform sensed mechanical force into an electrical signal. From theelectrical signal, a stress-strain curve of the flexible member can becalibrated and the flexible member can be used in conjunction with thesensor in a stand alone capacity to measure forces directed across theflexible member.

To calibrate a flexible member using strain gauges, the strain gaugesare usually coupled on opposite sides of the flexible member. Theflexible member is then deflected by a displacement generator over apreset range. The stress-strain curve of the flexible member iscalculated from the preset displacement and the measured load which isderived from the electrical signals generated by the strain gauges.

Similarly, flexible members can be calibrated through use of load cellsand other force transducers. The flexible member is operably coupled tothe load cell such that a load is applied thereto when the flexiblemember is displaced. The load and displacement are then used tocalculate a stress-strain constant. Again, the flexible member and loadcell can then be removed from the test apparatus and used to measureunknown forces.

However, conventional sensors, such as strain gauges and load cells, arenot sensitive enough to measure very small forces, e.g., less than 0.1grams, directed across the flexible member. Thus, the sensor andflexible member can not be used to measure minute forces, such as waterdrag on sea-going vessels or in flow tanks, air pressure from ducts andover surfaces and frictional forces in bearings. Furthermore, knowncalibration techniques are limited by stiction between components whichis difficult to measure, eliminate, or account for when measuring minuteforces. Also, many calibration techniques are restricted to utilizingforces directed in only a limited range due to system torque.

Therefore, it is desirable to provide an apparatus enabling simple andaccurate calibrating of a flexible member. It is further desirable toprovide an apparatus capable of calibrating a flexible member capable ofmeasuring forces less than one tenth of a gram. Furthermore, it isdesirable to provide an apparatus capable of redirecting forces to thedirection of the force being measured.

SUMMARY OF THE INVENTION

The present invention is directed at providing an apparatus formeasuring minute forces in any direction. Accordingly, the apparatusincludes a non-extensible element suspended between a pair of supports.The non-extensible element is operably coupled to a flexible member fortranslating forces applied to the non-extensible element to the flexiblemember. A pair of equally massed weight dishes are suspended from saidnon-extensible element on opposite sides of the flexible member. Byincrementally adding weights to one of the weight dishes, the flexiblemember is incrementally displaced. An Eddy Current System operablycommunicates with the flexible member for detecting the displacement.The displacement and load are used to calibrate a stress-strain curve ofthe flexible member.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to appreciate the manner in which the advantages and objects ofthe invention are obtained, a more particular description of theinvention will be rendered by reference to specific embodiments thereofwhich are illustrated in the appended drawings. Understanding that thesedrawings only depict preferred embodiments of the present invention andare not therefore to be considered limiting in scope, the invention willbe described and explained with additional specificity and detailthrough the use of the accompanying drawings in which:

FIG. 1 is a schematic view of a force-sensing apparatus for measuringminute forces in any direction according to the present invention;

FIG. 2 is an enlarged sectional view of the force-sensing apparatus ofFIG. 1 according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an apparatus for measuring forces in any directionis shown generally at 1. Although the apparatus 1 is particularlywet-suited for measuring forces less than one-tenth of a gram, it is tobe understood that the present invention can be used for measuringforces in a much greater range. As is generally known, a sensor andflexible member can be used to measure forces directed across theflexible member. To accomplish this however, a stress-strain curve mustbe calculated for the flexible member by calibrating the sensor andflexible member.

A stress-strain value of a flexible member may be calculated by applyinga known force to the flexible member and measuring its displacement.Furthermore, the stress-strain value is able to be calculated bydisplacing the flexible member a present mount and measuring the load.The load and displacement are then used to plot a stress-strain curvefor the flexible member. After the calibration, the flexible member canbe utilized for measuring unknown forces directed across it by measuringits displacement under the unknown load and referring to thestress-strain curve for calculating force.

In calculating the stress-strain curve of the flexible member, it iscritical to account for any torque in the system. The present inventionallows accurate and simple calculations of force at very small loadlevels, e.g., less than one-tenth of a gram, or when the displacement ofthe flexible member is very small, e.g., 0.001-0.040 inches. Prior tothe invention hereof, this has been difficult and the accuracy of thecalculations has been jeopardized.

Still referring to FIG. 1, the force calibrating apparatus 1 includes afirst upwardly-standing support stand 10 and a second upwardly-standingsupport stand 12. The second upwardly-standing support stand 12 isdisposed a predetermined distance away from the first support 10 and anon-extensible element 14 is suspended therebetween.

Preferably, each of the first and second supports 10, 12 includes avertically projecting elongated rod 16 and a base 18. The rod 16 issecured to the base 18 at a distal end 20 and extends verticallytherefrom to a proximal end 22. It should be appreciated that a ringstand is preferred for this purpose since it does not bend, flex, orlean when subjected to predetermined loads.

A generally C-shaped clap 24 is removably secured to the rod 16 suchthat it may be variably secured at different locations along the rod 16.The clap 24 includes a slot 26 therein extending from a top surface 28to a bottom surface 30 thereof. The slot 26 is adapted to receive therod 16 therethrough and orient the clamp 24 partially circumferentiallyabout the rod 16.

The clap 24 further includes a lateral Opening 32 therein for removablyreceiving a threaded member 34 therethrough. It has been found that athumb screw is particularly well suited for this purpose. The lateralopening 32 is located adjacent the slot 26 so that the threaded member34 may frictionally engage the rod 16. It should be noted that thespecific clap 24 used is not critical so long as it adequately supportsthe non-extensible element 14 to be described in greater detail below.

The non-extensible element or string 14 is secured at a first end 36 tothe first support member 10 and at a second end 38 to the second supportmember 12. Preferably, the string 14 is supported at its first andsecond ends 36, 38 by the clamps 24. In this way, the string 14 isvertically adjustable along the rods 16 by moving the clamps 24. Thelength of the string 14 and the distance between the supports 10, 12 arecoordinated such that the non-extensible string 14 is suspendedtherebetween.

Although other substitute non-extensible elements 14 are readilyavailable, in the preferred embodiment of the present invention, a ballchain non-extendible string is utilized. By using a ball-chain, thestring 14 is not only non-extensible, but is also easily secured to thefirst and second support members 10, 12 and can be readily shortened orlengthened as required. Also, the troughs between subsequent balls alongthe string provide convenient locations for securing the weight dishes40, 42 described in greater detail below.

A linearly flexible member 44, such as a metal plate, vertically extendsfrom a stand 45 and is disposed along the expanse of the non-extendiblestring 14 between the supports 10, 12. Preferably, the flexible member44 is located approximately at the mid-point between the first andsecond support members 10, 12 and is operably coupled to thenon-extensible string 14 such that forces applied to the non-extensiblestring 14 are translated across the flexible member 44. Preferably, theflexible member 44 linearly deforms proportionately to the forcedirected thereon over a desired measurement range.

A first force applicator or weight dish 40 is suspended along thenon-extensible string 14 between the flexible member and the firstsupport stand 10. Additionally, a second force applicator or weight dish42 is suspended along the non-extendible string 14 between the flexiblemember 44 and the second support stand 12. Preferably, the weight dishes40, 42 include a cup-shaped dish 46 having a bottom surface 48 andupwardly projecting side walls 50 defining a void 52 for holding variousweights (not shown).

The cup-shaped dish 46 is secured to three hanging arms 54 equidistantlyspaced about a perimeter thereof. The hanging arms 54 extend upwardlyfrom the cup-shaped dish 46 to an intersection point 56 above the bottomsurface 48. A hook 58 is secured to the intersection 56 and upwardlyprojects therefrom. The hook 58 is adapted to overhang the string 14such that the cup-shaped dish 46 is suspended therefrom. In this way,the weight dishes 40, 42 hang from the string 14 with the bottom surface48 essentially horizontal so as to hold weights without spillage.

Although the weight dishes 40, 42 shown and described are preferred, itshould be noted that many other designs are acceptable for this purpose.Preferably, the first weight dish 40 and the second weight dish 42 haveequivalent mass when empty. In this way, when the empty weight dishes40, 42 are suspended from the string 14 and the flexible member 44 issecured at the midpoint therebetween, zero force is applied across theflexible member 44. If the weight dishes 40, 42 have unequal mass, thesystem 1 must be balanced by adding weight to one weight dish 40, 42 orvarying the location of the flexible member 44 between the supportstands 10, 12.

A displacement generator 60 such as a micrometer is disposed between theflexible member 44 and the first support stand 10. It should be notedthat the displacement generator could equally as well be placed betweenthe flexible member 44 and the second support stand 12. The micrometer60 is supported on a stand 62 and is secured at a first end 64 of amovable carriage 66 to the flexible member 44. A force transmitting arm68 is rotatable to move the moveable carriage 66 thereby displacing theflexible member 44. A high-resolution Eddy current system 70 such as theKaman KDM-7200-IUEP system, is operatively coupled to the flexiblemember 44 for detecting very small displacements thereof. Initially, themicrometer 60 is used in conjunction with the Eddy current system 70 toverify that the stress strain curve of the flexible member 44 is linearover an expected range.

As shown in FIG. 2, the force applied parallel to F₁ (T₂) in FIG. 1, canbe varied by any angle (θ₂) and the force value remains able to becalculated. As can be seen, the angle θ₁ is defined between a firstsection 72 of the non-extensible string 14 and a horizontal axis 74. Theangle θ₂ is defined between the horizontal axis 74 and a second section76 of the non-extensible string 14. A weight W represents the mass addedto the weight dish. Thus, a force T₂ is able to be calculated throughthe formula T₂ =W/[cosθ₂ tanθ₁ - sinθ₂ ]. It should be noted that in thepreferred embodiment, as shown in FIG. 1, the angle θ2 equals 0 and theangle θ1 equals forty-five degrees. Therefore, the force T₂ equals themass W.

The operation of the present invention will now be described in greaterdetail. In an initial mode, the first and second weight dishes 40, 42are suspended from the non-extensible string 14. Since the weight dishes40, 42 are equal in mass and are equidistant from the flexible member44, zero force is placed across the flexible member 44. In a test mode,the force transmitting arm 68 of the displacement generator 60 isrotated across a pre-determined range to cause the moveable carriage 66to move axially. The axial movement of the moveable carriage 66displaces the flexible member 44 over a given range. The mechanicalmovement of the flexible member 44 is converted to electrical signals bythe Eddy current system 70 and the volts per inch displayed thereon arechecked to insure that they remain constant. In this way, the EddyCurrent System 70 is used to verify that the displacement of theflexible member 44 is linear.

In a calibration mode, the flexible member 44 is returned to a normal,non-flexed position and preset weights are incrementally added to thesecond weight dish 42. By adding weight to the second dish 42 inincremental mounts, an increasing force F₁ is exerted in a firstdirection. The incremental weight displaces the flexible member 44 andgenerates a curve of force-to-volts on the Eddy Current System 70.Preferably, a high resolution display 78 is available on the EddyCurrent System 70 for conveying the curve to the operator. The data isthen combined to produce a stress-to-strain curve for the flexiblemember 44, which should be linear.

In a measuring mode, the flexible member 44 and Eddy Current System 70are removed from the apparatus 1 and used to measure forces directedacross the flexible member 44. To accomplish this, the flexible member44 is positioned within a system to be measured and the displacementthereof generates a value on the Eddy current system 70 which iscompared to the stress-strain data. From the comparison, a valuecorresponding to the force across the flexible member 44 is determined.

The present invention has been found capable of measuring forces evenless than a tenth of a gram in any plane, without subjecting theflexible member 44 to out of plane torques. This accuracy has beenmaintained even when displacements are minute, such as 0.001-0.040inches. The suspension of the weight dishes 40, 42 redirects smallforces through the non-extensible string 14 into the direction of forcemeasurement. This is superior to other methods which are limited bystiction. The suspension method also allows force measuring in anydirection. Furthermore, no torque is applied out of the direction of theforce being measured as along as the strain accompanying the force issmall relative to the non-extensible string 14 length.

Accordingly, the present invention provides an apparatus for simply andaccurately measuring forces across a flexible member. The presentinvention is particularly well suited for measuring forces less thanone-tenth of a gram. Furthermore, the present invention re-directs smallforces in the direction of the force being measured and is not limitedby stiction. Moreover, the present invention allows force measurement inany direction without torques being applied out of the direction of theforce being measured.

Those skilled in the art can now appreciate from the foregoingdescription that the broad teachings of the present invention can beimplemented in a variety of forms. Therefore, while this invention hasbeen described in connection with particular examples thereof, the truescope of the invention should not be so limited since othermodifications will become apparent to the skilled practitioner upon astudy of the drawings, specification, and following claims.

What is claimed is:
 1. An apparatus comprising:a suspendednon-extensible element; a flexible member coupled to said non-extensibleelement; a plurality of force applicators suspended from saidnon-extensible element for displacing said flexible member by applying aforce to said non-extensible element; and an Eddy Current Systemoperably communicating with said flexible member for detecting saiddisplacement.
 2. The apparatus of claim 1 wherein said non-extensibleelement further comprises:a first end secured to a first support; and asecond end secured to a second support; whereby said first support andsaid second support suspend said non-extensible element therebetween. 3.The apparatus of claim 2 wherein said non-extensible element furthercomprises a ball chain.
 4. The apparatus of claim 1 wherein saidplurality of force applicators comprise:a first weight dish suspendedfrom said non-extensible element on a first side of said flexiblemember; and a second weight dish suspended from said non-extensibleelement on a second side of said flexible member.
 5. The apparatus ofclaim 4 wherein a mass of said first weight dish is equal to a mass ofsaid second weight dish.
 6. The apparatus of claim 1 furthercomprising:a displacement generator operably secured to said flexiblemember for displacing said flexible member a pre-determined amount. 7.The apparatus of claim 6 wherein said displacement generator comprises amicrometer.
 8. An apparatus for measuring force directed across aflexible member comprising:a first vertically extending support; asecond vertically extending support displaced from said first support; anon-extensible element secured at a first end to said first support andsecured at a second end to said second support so as to be suspendedtherebetween; said non-extensible element being secured to said flexiblemember for translating a force applied to said non-extensible string tosaid flexible member; a first weight dish suspended from saidnon-extensible element a given distance from said flexible member on afirst side of said flexible member; a second weight dish suspended fromsaid non-extensible element said given distance from said flexiblemember on another side of said flexible member; a displacement generatoroperably secured to said flexible member for displacing said flexiblemember a pre-determined amount; and an Eddy Current System operablycommunicating with said flexible member for measuring said displacementof said flexible member.
 9. The apparatus of claim 8 wherein said firstsupport and said second support comprise ring stands.
 10. The apparatusof claim 8 wherein said non-extensible string comprises a ball chain.11. The apparatus of claim 8 wherein said flexible member comprises anupwardly projecting metal plate.
 12. The apparatus of claim 8 whereinsaid displacement generator comprises a micrometer.
 13. The apparatus ofclaim 8 wherein a mass of said first weight dish is equal to a mass ofsaid second weight dish.
 14. A method of calibrating a stress-straincurve of a flexible member comprising:securing said flexible member to asuspended non-extensible element; incrementally suspending weights fromsaid suspended non-extensible element; measuring an incrementaldisplacement of said flexible member as said weights are incrementallysuspended from said non-extensible element by an Eddy Current Systemoperably communicating with said flexible member; and incrementallycombining a mass of said weights and said displacement to determine saidstress-strain curve.